Flucating Engine Temperature

[VTsaab]

Hi Folks:

I'm new to the forum, I've been lurking for a while and finally decided to join because of an annoying issue with engine temperature. As I said in the title, I have been having issues with a flucuating engine temp, I have switched thermostats 3 times and the temperature sensor once and am still having issues with the temperature. The gauge is indicating that the engine is running cool and the engine does cool as the gauge indicates, at least by the temp of the air blowing out the heat vents in the cabin. The top radiator hose also indicates the thermostat is opening immediately because it starts to warm immediately after starting the car. I can't believe that I have installed 3 bad thermostats. I have also done a combustion leak test to the coolant overflow tank thinking that it might be a head gasket on the way out but that showed to be negative. Any thougts ?
Thank you

[Burnsside42]

Have you thought of air in the cooling system? If there is an air bubble stuck somewhere and it can't get out - you will get strange problems. Get the system flushed and bled is my suggestion.